This invention relates in general to a process for fabricating a semiconductor device, and more particularly to a plasma dry etching process. 1. Background
The current practice in semiconductor manufacturing is to use thin film fabrication techniques. Etching, in semiconductor applications, is a fabrication process used to produce patterns by which material is removed from the silicon substrate or from thin films on the substrate surface. A mask is typically used to protect desired surface regions from the etchant and this mask is stripped after the etching has been performed. The composition and uniformity of these thin layers must be strictly controlled to facilitate etching of submicron features.
As the pattern dimensions approach the thicknesses of the films being patterned, it is increasingly required that a dry etching process be selected. The basic concept of plasma dry etching is rather direct. A plasma is defined to be a partially ionized gas composed of ions, electrons, and a variety of neutral species. A glow discharge is a plasma that exists over a pressure range of 1 mtorr to 5.6 torr, containing approximately equal concentrations of positive particles (positive ions) and negative particles (electrons and negative ions). In plasma etching applications the glow discharge can be used to produce energetic ionic bombardment of the etched surface. However, the glow discharge has another even more important role, that of producing reactive species, such as atoms, radicals and ions from a relatively inert molecular gas, for chemically etching the surfaces of interest. The gases adopted for plasma etching processes have traditionally been selected on the basis of their ability to form reactive species in plasma, which then react with the surface materials being etched and lead to volatile products. Conventional dry etching processes require the use of hazardous, carcinogenic and/or toxic gases.
Plasmas consisting of fluorine-containing gases, such as carbon tetrafluoride (CF.sub.4), are extensively used for etching Si, SiO.sub.2 and other materials in semiconductor fabrication. When a glow-discharge exists, some fraction of the CF.sub.4 molecules are dissociated into other species. In addition to CF.sub.4 molecules, ionic species, and electrons, there are a large number of radicals that are formed. A radical is an atom, or collection of atoms, which is electrically neutral, but which also exists in a state of incomplete chemical bonding, making it very reactive.
The radicals, in fact, are responsible for most of the actual chemical etching phenomena that occur at the surface of the material being etched (except for the etching of Al, which is apparently etched by molecular Cl.sub.2).
In addition to CF.sub.4 many different gases are used in dry etching processes for semiconductor fabrication, either individually or as the components of the mixtures of several gases. Chlorine plasmas etch polysilicon very anisotropically and exhibit excellent selectivity over SiO.sub.2 but they etch Si more slowly than flourine containing gases.
Controlled anisotropic etching of Si with flourine-based plasmas is difficult and these plasmas exhibit undesirable characteristics in etching processes. Additionally, etch profiles vary widely with the processes parameters, especially the gas composition. Thus etch gases containing both chlorine and fluorine have come to be preferred for polysilicon etching. These etch gases are most often introduced as chloroflourocarbon (CFC) molecules such as freon.TM..
Molecules like carbon tetrachloride (CCl.sub.4) and carbon tetraflouride (CF.sub.4) are used because in a plasma reaction they produce the desirable reactive species which,in various combinations with other gases, etch silicon, silicon oxides, and other materials used in the manufacture of integrated circuits. Heavy hydrogenated analogs of these molecules like methyl chloride (CH.sub.3 Cl) or methyl flouride (CH.sub.3 F) tend to form polymers which interfere with the desired etch reaction. With the increased awareness of the effects CFC's have on the environment and concommitant health and safety restrictions and regulations, it is clear that semiconductor manufacturers can no longer rely on chloroflourocarbon molecules to provide the reactive species in plasma etching.
What is still needed is a dry plasma etch process that utilizes non-CFC compounds to provide the necessary reactive species for the semiconductor fabrication etch process.